The present invention relates to a monomode fiber ring interferometric device.
In a ring interferometer, or Sagnac interferometer, two beams travel in opposite directions over the same optical path and interfere with each other at the exit from this path. As long as a disturbance of this path has the same characteristics for both directions of propagation and does not vary during the transit time of the light in the interferometer, the two beams are affected identically and their relative phase remains unchanged. Disturbances of this type are called "reciprocal". Because the transit time in an interferometer is generally very short, the variations of a disturbance during this time, unless it is introduced voluntarily, are generally negligible.
But there exist "non reciprocal" disturbances which have a different amplitude in the two directions of propagation, it is a question of physical effects which, by establishing its complete orientation, destroy the symmetry of the space or of the medium.
Two known effects have this property:
the Faraday effect, or colinear magneto-optical effect, whereby a magnetic field creates a preferential orientation of the spin of the electrons of the optical material; PA1 and the Sagnac effect, or relativist inertia effect, where the rotation of the interferometer with respect to a Galilean reference destroys the symmetry of the propagation times.
The use of the rotation with respect to inertial space leads to the construction of optical fiber gyrometers or rate meters and the use of the magnetic field leads to the construction of amperometric current sensors or magnetometers.
It has been demonstrated that the use of a particular so called reciprocal configuration cancels out exactly any phase shift other than those induced by the non reciprocal effects.
This configuration however requires the light to be detected coming back through the monomode entrance gate of the interferometer. This requires using a separator which sends a part of the exiting light to a detector while coupling light at the entrance. The return signal to the detector is optimum when the separator is of the 50--50 type but that causes a systematic loss of 6dB and adds a component to the system. It has been proposed replacing the separator by a high speed switch which is placed in a first switching state when the light is emitted by the source and transmits it to the system at the entrance and which is placed in its other switching state when the light returns for sending it to the detector. Since the fiber lengths used in these systems are of the order of 200 meters to 2 kilometers, the delays due to the propagation are of the order of 1 to 10 microseconds.
Such a device is described in the French patent FR-B-2 409 518. The energy balance is improved, but the architecture of the device may be further simplified.